1. Field of the Invention
The invention relates to a process for producing precision steel tubes by cold forming of tube blanks, in particular in a plurality of forming steps, with or without an internal tool.
2. Description of the Prior Art
A process for producing precision steel tubes by call forming of tube banks is known from DE 38 14 648 A1.
For many years, it has been known in the general prior art to reduce the dimensions (diameter and wall thickness) of seamless or welded steel tubes further by cold forming, to produce precision tubes. In doing so, it is possible to work with or without an internal tool. The most widespread method is cold forming by cold drawing. However, it is also possible, for example, for the tube blanks used as starting material to be formed to the desired dimensions by cold pilger rolling. The latter method is usually limited to the working of stainless steels and special alloys. Tube blanks made from structural steels are predominantly used in particular during cold drawing.
Changes in the technological properties often occur during the cold forming of steels. Depending on the degree of deformation, there are significant increases in the tensile strength (cold work hardening). For structural steels, the increase in tensil strength is inevitable because of the carbon content and nitrogen content of the steel (significantly over 0.005%), considerable deterioration in the ductility (e.g. elongation at break) accompanies the increase in tensile strength. The cold work hardening, which is often highly desirable in the end product, represents a considerable drawback for the cold forming itself, since as the extent of cold forming increases ever greater forces have to be applied for further forming, so that the performance limits of the forming machine employed may be exceeded. Furthermore, the decreasing ductility imposes limits on the further cold formability. For these reasons, it is normally necessary to carry out a microstructural transformation by a normalizing treatment after one or two cold forming steps. Before the next cold forming step can take place, it is necessary to form a drawing compound layer and a drawing compound carrier layer. This considerably increases the outlay involved in producing precision steel tubes.
U.S. Pat. No. 5,200,005 has disclosed a process which allows the production of sheets of increased strength and ductility by hot forming from a so-called IF (Interstitial steel). If steels have been known for many years for the production of deep drawing sheets. In a first variant, U.S. Pat. No. 5,200,005 provides for the production of steel strip by roughing rolling at a temperature of 1260.degree. C. and finishing rolling at approximately 710.degree. C. According to a second process variant, the roughing rolling takes place at 850.degree. C. and the finishing rolling takes place at approximately 700.degree. C.
Hitherto, there has been no disclosure of steel tubes being produced from IF steels. On the one hand, this may well result from the fact that IF steels inherently have a comparatively low strength. On the other hand, such steels cannot be worked by hot rolling under the standard process conditions for producing seamless steel tubes, since their deformation resistance is too low. The production of welded tubes from IF steel has not been considered either hitherto.